The replaceitem command cannot replace a elytra on the 'chest' slot on a player (Might do the same for other mobs too, but not tested)

How to replicate:

1. type this command: /replaceitem entity @p slot.armor.chest minecraft:elytra

2. You get this message: Could not replace slot 102 with 1 * [Elytra]

The same is for command blocks